In a wireless mesh network, all wireless stations (nodes) in the mesh network are equal so far as data communication is concerned. Multicast technology used to deliver data among a group of nodes. Two existing multicast technologies for wireless ad hoc network (wireless mesh network) are Multicast operations in Ad hoc On-Demand Distance Vector (AODV) routing protocol (MAODV) and tree-based multicast algorithm included in an IEEE 802.15.5/D2 draft specification (IEEE P802.15.5/D2, “Draft Recommended Practice to STANDARD [FOR] Information Technology-Telecommunications and information exchange between systems-Local and Metropolitan Networks-Specific requirements-Part 15.5: Mesh Enhancements for IEEE 802.15 WPANs”, Dec. 17, 2007).
The MAODV protocol is intended for use by mobile nodes (mobile wireless stations) in an ad hoc network. MAODV offers quick adaptation to dynamic link conditions, low processing and memory overhead, and low network utilization. It creates bi-directional shared multicast trees connecting multicast sources and receivers. These multicast trees are maintained as long as group members exist within the connected portion of the network. Each multicast group has a group leader whose responsibility is to maintain the group sequence number, used to ensure freshness of routing information.
Route Requests (RREQs), Route Replies (RREPs), Multicast Activations (MACTs) and Group Hellos (GRPHs), are message types utilized by the multicast operation AODV. RREQs and RREPs are handled as specified in the unicast AODV (RFC 3561), except for certain procedures specifically designed for multicast.
As long as the multicast group members remain connected (within a “multicast tree”), MAODV does not play any role. When a node either wishes to join a multicast group or find a route to a multicast group, the node uses a broadcast RREQ to discover a route to the multicast tree associated with that group. For join requests, a route is determined when the RREQ reaches a node that is already a member of the multicast tree, and the node's record of the multicast group sequence number is at least as great as that contained in the RREQ.
For non-join requests, any node with a current route to the multicast tree may respond to the RREQ. A current route is defined as an unexpired multicast route table entry whose associated sequence number for the multicast group is at least as great as that contained in the RREQ. The route to the multicast tree is made available by unicasting a RREP back to the source of the RREQ. Since each node receiving the request caches a route back to the source of the request, the RREP can be unicast back to the source from any node able to satisfy the request. Once the source node has waited the discovery period to receive RREPs, it selects the best route to the multicast tree and unicasts the next hop a MACT message along that route. This message activates the route.
Nodes monitor the link status of next hops on the multicast tree. When a link break on the multicast tree is detected, the tree branch should be immediately repaired through the use of the RREQ/RREP/MACT messages.
A multicast group leader is associated with each multicast group. The primary responsibility of this node is the initialization and maintenance of the group sequence number. A Group Hello message is periodically broadcast across the network by the multicast group leader. This message carries a multicast group and group sequence number and corresponding group leader IP address. This information is used for disseminating updated group sequence numbers throughout the multicast group and for repairing multicast trees after a previously disconnected portion of the network containing part of the multicast tree becomes reachable once again.
The tree-based multicast routing protocol in the said IEEE 802.15.5/D2 draft specification, utilizes a logical tree built by the unicast routing protocol of the standard. The logical tree is a shared tree rooted at the Mesh Coordinator (MC) of the mesh network. When the tree is built, the neighbor information (up to N-hops away) as well as its relationship (parent, child, or sibling) to a node is recorded in every node's neighbor list.
The goal of tree-based multicast routing is defined as finding a minimum sub-tree of the logical tree which covers all multicast members of a multicast group. Joining and leaving the multicast group is dynamic so the multicast tree is minimal at any time during the multicast session. Because of the use of the tree structure, all control messages are unicast and no multicast routing table is needed. In most cases, the MC is not bothered for transmitting control and data messages hence the congestion around MC and single-point-of-failure problem can be avoided or relieved.
Furthermore, multicast data frames do not need to be sent to the MC first for delivery. They can be propagated to all other members directly from the data sources to ensure simple and timely data delivery. Non-members can also send packets to members but not vice versa. The multicast routing protocol uses six command frames for multicast tree formation and membership management as:
Group - Join REQuest (G-JREQ)Group - Join REPly (G-JREP)Group - Leave REQuest (G-LREQ)Group - Leave REPly (G-LREP)Group - Coordinator UpDate (G-CUD)Group - DISmiss (G-DIS)